<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      const arr = [2, 3, 4]
      const obj = { username: 1, password: 123 }

      // for (let key in arr) {
      //   console.log(key)
      // }

      // for (let key of arr) {
      //   console.log(key)
      // }

      //   for (let key of arr) {
      //     console.log(key)
      //   }

      for (let key of '123') {
        console.log(key)
      }
    </script>
  </body>
</html>
